The objectives of this project are to: a) house mice, infect them with murine viruses, care for the mice after infection, record mortality and perform tests for viral infection; b) prepare bone marrow chimeric mice and to house these mice until needed by the investigator; c) breed and house and immunologically manipulate mice that are immune deficient such as athymic nude mice, SCID mice, or mice undergoing a graft-versus-host reaction; and d) perform mouse embryo transfer into pseudopregnant mice and to perform genetic manipulations on the embryos prior to transfer of the embryos to the pseudopregnant mice.